1. Field of the Invention
The present invention relates to systems for transmitting data across a communication channel, and more particularly to a method and an apparatus for adaptively equalizing a signal flowing through a communication channel to compensate for transmission characteristics of the communication channel.
2. Description of Related Art
In digital data transmission systems, data in digital form is transmitted over media, such as wires or fiber optic cables, from a transmitter to a receiver. The digital data waveform is distorted with respect to instantaneous frequency and amplitude as it propagates along the transmission media due to electrical noise, dispersion and the unique frequency response of the transmission medium. Electrical noise refers to the unwanted components of an electrical signal that tend to disturb accurate transmission and processing of the signal. Dispersion refers to pulse spreading of the signal, and is measured in units of distance per time of travel.
Receivers in a digital data transmission system ideally regenerate the transmitted pulse train to its original form, without error. In digital receivers, this reconstruction is typically achieved by sampling the pulse train at a regular frequency, and at each sample instant making a decision as to the most probable symbol being transmitted. Typically, a threshold level is chosen to which the received signal is compared. Above this threshold level, a binary one is registered and below the threshold level a binary zero is registered. Errors may occur in the analog to digital conversion process due to various noises and disturbances. The noise sources can be either external to the system, for example, atmospheric noise or equipment generated noise, or internal to the system. Internal noise is present in every communications system and represents a basic limitation on the transmission and detection of signals.
The amplitude of the received signal may be degraded to the point where the signal to noise ratio at the decision instant may be insufficient for accurate decisions to be made consistently. For instance, with high noise levels, the binary zero may occur above the threshold and hence be registered as a binary one. Moreover, the actual received data transmissions may be displaced in time from the true transmission. This displacement, or intersymbol interference ("ISI"), of the transition is caused by a new pulse arising at the receiver before the previous pulse has died down. Intersymbol interference occurs due to pulse spreading caused by the dispersion of the transmission media. Variations in the clock rate and phase degradations also distort the zero crossings resulting in decision time misalignment. When a pulse is transmitted in a given timeslot, most of the pulse energy will arrive in the corresponding timeslot at the receiver. However, because of pulse spreading induced by the transmission medium, some of the pulse energy will progressively spread into adjacent timeslots, resulting in an interfering signal. The effect of pulse spreading may be reduced by equalization, which provides a frequency dependent gain and phase delay to force the transmitted binary one to be followed by zero at all neighboring decision times to confine as much of the pulse energy as possible to the timeslot to which it corresponds. The purpose of equalization, then, is to mitigate the effects of signal degradation and intersymbol interference. The equalizer thus optimizes the signal at the decision instant so that the bit error rate of the system may be minimized. Equalizers are basically filters with frequency and phase responses that are typically inverse of that of the transmission medium.
Adaptive equalization involves adjusting the transfer function or frequency response of the equalizing filter continuously during data transmission. Some of the known adaptive equalization methods include switching capacitor techniques and digital signal processing techniques. Both of these methods require digital signal sampling at least two times the Nyquist rate or as much as between eight and twelve times the transmitted data rate. Such a high sampling rate makes these methods difficult to apply in high speed applications. They also require large amounts of circuitry, which translates into higher power consumption.
These equalizing filters used for adaptive equalization have a range of settings corresponding to various lengths of transmission medium. The receiver needs to figure out correctly and accurately the channel characteristics on the transmission medium the signal has been through so that it can chose the appropriate filter setting to undo the effect of the transmission medium. This process is called tuning or "adapting" the equalizer.
Furthermore, existing adaptive equalization systems often reach metastable states, and do not converge to the optimal equalization and amplification settings. Additionally, existing high speed adaptive equalization systems are implemented in expensive bipolar technology, and not inexpensive complimentary metal oxide semiconductor ("CMOS") technology or by combinations of digital signal processors ("DSP") with specialized CMOS logic.
Some digital data transmissions use a ternary coding scheme known as MLT3. MLT3 uses three signal values: 1, 0 and -1, and ensures that groups of ones that are separated by zeros will always alternate in polarity. In other words, data patterns such as 1, 1, 1, 0, 0, 0, 1, 1, 1 and -1, -1, -1, 0, 0, 0, -1, -1, -1 are illegal because the ones do not alternate in polarity.
What is needed is a system for adaptively equalizing a digital data transmission which rapidly converges to optimal equalization and amplification settings without getting caught in a metastable state. Additionally, what is needed is an adaptive equalization system which can be implemented in CMOS technology, with minimum power, area and complexity.